THE WORK OF RUNNING WATER 61 



flows from a plateau 2,000 feet above the sea and 200 miles from it, 

 by a direct course, it has an average fall of 10 feet per mile; but if 

 it runs off a plateau of equal height 2,000 miles from the sea, it has 

 an average fall of one foot per mile. If the volume of the stream 

 is the same in the two cases, the valley in the plateau nearer the sea 

 will become much deeper than the other. Valleys near the borders 

 of continents are therefore likely to be deeper than those in the 

 interiors of the continents, in lands of the same elevation. 



A stream cuts the lower end of its channel down to about the 

 level of the lake, sea, or river into which it flows. The level of the 

 body of water into which a river flows therefore determines the level of 

 its channel; but a valley reaches the level of the water to which it 

 leads only at its lower end. Its upper part is always higher. 



The widening of valleys. If the growth of a valley were due 

 merely to the down-cutting of the stream, the valley would be no 



Fig. 47. Diagram of a valley, the top of which is ten times the width of the 



stream. 



wider than the stream which flows through it (Fig 47) . Since most 

 valleys are much wider than their streams, something besides the 

 down-cutting of the streams must help along their growth. The 

 widening of valleys is brought about in many ways, among them 

 the following: 



(1) A stream sometimes flows against one side of its channel with 

 such force as to under-cut the slope above (PL VIII, p. 13, and Fig. 

 48). Slow streams are more apt to widen their valleys in this way 

 than swift ones, because slow streams are more easily turned against 

 their banks by obstacles in the channels. 



(2) The rain-water flowing down the slopes of a valley carries 

 mud, sand, etc., with it. This also widens the valley, by slowly 

 wearing back its slopes. 



(3) 'The loose earthy matter which lies on the slopes of a valley 

 creeps slowly downward, especially when wet. From a steep valley 

 slope, it may, under some conditions, slide or slump down (Fig. 29) . 



